Oxygen is an economically important gas which is widely used in large-scale industrial applications. New uses for oxygen are emerging in advanced high-temperature processes for iron and steel manufacture, coal gasification, integrated gasification combined cycle power generation, and oxygen-enriched combustion. In these large-scale applications, the cost of oxygen produced by conventional cryogenic or noncryogenic technology is a major portion of the overall operating cost, and lower oxygen cost will encourage the commercialization of these emerging technologies. New oxygen separation processes which can be thermally integrated with these advanced high-temperature processes will reduce the energy consumed in oxygen production, which in turn will promote the technical and commercial development of such integrated systems.
Oxygen can be recovered from air at high temperatures by inorganic oxide ceramic materials utilized in the form of selectively permeable nonporous ion transport membranes. An oxygen partial pressure differential or a voltage differential across the membrane causes oxygen ions to migrate through the membrane from the feed side to the permeate side where the ions recombine to form electrons and oxygen gas. An ion transport membrane of the pressure-driven type is defined herein as a mixed conductor membrane, in which the electrons simultaneously migrate through the membrane to preserve internal electrical neutrality. An ion transport membrane of the electrically-driven type is defined herein as a solid electrolyte membrane in which the electrons flow from the permeate side to the feed side of the membrane in an external circuit driven by a voltage differential. A comprehensive review of the characteristics and applications of ion transport membranes is given in report entitled "Advanced Oxygen Separation Membranes" by J. D. Wright and R. J. Copeland, Report No. TDA-GRI-90/0303 prepared for the Gas Research Institute, September 1990.
The operation and heat integration of ion transport membranes with combustors, gas turbines, and power recovery systems are disclosed i n U.S. Pat. Nos. 4,545,787, 5,035,727, 5,118,395, and 5,174,866. An article entitled "Separation of Oxygen by Using Zirconia Solid Electrolyte Membranes" by D. J. Clark et al in Gas Separation and Purification 1992, Vol. 6, No. 4, pp. 201-205 discloses an integrated coal gasification-gas turbine cogeneration system with recovery of oxygen for use in the gasifier. Nonpermeate from the membrane is combusted with gas from the gasifier and passed to the gas turbine cogeneration system.
Each of the high-temperature, nonporous, ion transport membrane systems summarized above is characterized by the use of a single membrane stage for oxygen recovery, and the use of multiple stages or zones is not disclosed.
The separation of gas mixtures by nonporous polymeric membranes and by porous diffusion membranes is well-known in the art. Membrane separation systems utilizing these principles operate at ambient or near-ambient temperatures, and are characterized by permeate selectivities which are significantly lower than the extremely high oxygen selectivities achieved by ion transport membranes. As a result, permeate streams are gas mixtures enriched i n certain components relative to the feed composition. Staged operation of nonporous polymeric membranes and porous diffusion membranes is disclosed in the art, wherein staged operation increases the purity and/or recovery of the desired product.
The successful development and commercialization of oxygen production by ion transport membranes will require flexible systems which maximize energy utilization and allow operation of system components at optimum conditions. The invention disclosed below and described in the claims which follow advances the art and provides improved methods for the production of oxygen by means of multiple ion transport membranes.